Switching power supplies are notorious for generating electrical noise. Isolated switching power supplies, however, have added electrical noise via displacement-current across an isolation barrier of the isolated switching power supply. Normally, isolation transformers are used to provide the isolation barrier between the input and output of the switching power supply. The design of the isolation transformer, however, can greatly impact the level of electrical noise within the switching power supply.
Most isolated switching DC-to-DC power supplies use isolation transformers that contain electrostatic winding-shielding between primary and secondary windings. The goal of these transformers is to have the voltage swings of the primary winding couple only to a primary winding-shield and the voltage swings of the secondary winding couple only to a secondary winding-shield; however, the transformers currently being used still create a large amount of electrical noise across the isolation barrier. The separation of the primary windings and the secondary windings results in high leakage inductance in the transformer. High leakage inductance often increases the electrical noise of a switching power supply.
A low leakage inductance transformer construction method is to wind a transformer using a bifilar winding technique in which two wires are wound next to each other at the same time. As the wire pair is repeatedly wound around a magnetic core, each turn of the wire pair couples to other turns that then lay upon previous turns. This additional coupling changes the leakage inductance and isolation capacitance. Small changes in the winding process can cause changes to these couplings. Thus the electrostatic coupling is not well controlled causing displacement current across the isolation barrier.
Therefore, there remains a need for improved isolation transformers. In an ideal transformer, the electrostatic coupling between the primary and secondary windings is only between the primary and secondary winding-shields. The two winding-shields voltage swings are the same and thus there is no displacement current between them. The voltage swings of the primary winding couple only to a primary winding-shield and the voltage swings of a secondary winding couple only to a secondary winding-shield. Also in an ideal transformer, the leakage inductance should remain low as in a bifilar wound primary/secondary transformer.